Apparatus for investigating earth formations including a flexible pad member having an investigating means and longitudinally extendible hinging means for rotating portions of the pad member



JQQ'HJJ w an RCH ROOM 0! 39379;965 SEA Apnl 23, 1968 o. R. TANGUY ETAL 3,379,965

APPARATUS FOR INVESTIGATING EARTH FORMATIONS INCLUDING A FLEXIBLE PAD MEMBER HAVING AN INVESTIGATING MEANS AND LONGITUDINALLY EXTENDIBLE HINGING MEANS FOR ROTATING PORTIONS OF THE PAD MEMBER Original Filed June 4, 1965 3 Sheets-Sheet l April 3, 1968 o. R. TANGUY ETAL 3,379,965

APPARATUS FOR INVESTIGATING EARTH FORMATIONS INCLUDING A FLEXIBLE PAD MEMBER HAVING AN INVESTIGATING MEANS AND LONGITUDINALLY EXTENDIBLE HINGING MEANS FOR ROTATING PORTIONS OF THE PAD MEMBER Original Filed June 4, 1965 I5 Sheets-Sheet 2 Apnl 23, 1968 o. R. TANGUY ETAL 3,379,965

APPARATUS FOR INVESTIGATING EARTH FORMATIONS INCLUDING A FLEXIBLE PAD MEMBER HAVING AN INVESTIGATING MEANS AND LONGITUDINALLY EXTENDIBLE HINGING MEANS FOR ROTATING PORTIGNS OF THE PAD MEMBER Original Filed June 4, 1965 3 Sheets-Sheet I5 l 1 F I I 6 6 o b 5 A "5/ A I so k l iy /A l i 47 v, I L 1 1 United States Patent APPARATUS FOR lNVEdTlGATING EARTH FOR- MATEONS INCLUDXNG A FLEXHELE PAD MEM- BER HAVING AN TNVESTIGATHNG MEANS AND LONGHUDINALLY EXTENDIBLE HINGENG MEANS FGR ROTATHN G PORTIONS OF THE PAD MEMQER Denis R. Tanguy, Houston, Dean F. Saurenman, Friendswood, and John E. Key, Houston, Tex., assignors to Schlumherger Technology Corporation, Houston, Tex., a corporation of Texas Continuation of application Ser. No. 461,392, June 4, 1965. This application June 28, 1967, Ser. No. 649,746

14 Claims. (Cl. 324-) ABSTRAQT OF THE DESCLOSURE In accordance with an illustrative embodiment of the invention, a wall-engaging pad member having earth formation exploring means is constructed in a manner that adjacent portions of the pad member can bend with respect to the central portion thereof. To accomplish this, a fluid-filled bladder is situated in a hollow chamber located between the different portions of the pad member. This fluid-filled bladder acts to maintain the adjacent pad portions in an outward position with respect to the central pad portion, but is displaceable in response to force being applied to the adjacent pad portions so that the pad member as a whole may more nearly conform to the radius of curvature of the borehole wall.

This is a continuation of application Ser. No. 461,392 filed June 4, 1965, now abandoned.

This invention relates to electrical apparatus for investigating subsurface earth formations traversed by a borehole and, more particularly, to apparatus for measuring the electrical resistance properties of a subsurface earth formation by means of electrodes in the borehole adapted to be pressed against the borehole wall.

One method of investigating subsurface earth formations traversed by a borehole is to move a system of pad mounted wall-engaging electrodes through the borehole and determine the resistance presented by the earth formations to the flow of electrical current emitted from one or more of the electrodes. The electrical record or log obtained from this manner of investigation aids in determining the nature and lithological character of the various subsurface earth formations. The resulting data is useful in the case of oil well boreholes in that, among other things, it enables the presence and depth of any oil or gas bearing strata to be determined.

In drilling a borehole by the usual rotary method, the borehole is filled with a drilling mud during the drilling process. This drilling mud will penetrate radially into the subsurface earth formations for a distance dependent upon the porosity of the earth formations. When the drilling mud invades a permeable strata, there is a mudcake remaining on the wall of the borehole. This mudcake is caused by the solid particles of drilling mud being unable to pass through the permeable strata. The mud filtrate which actually invades the permeable zone alters the electrical resistance properties of the earth formation immediately adjacent to the borehole wall and, not uncommonly, increases the resistance of such portions, particularly where fresh muds are used. However, the resistivity of the mud cake which is formed on the borehole wall is usually relatively low compared to the resistivity of the flushed zone.

For the case of pad mounted electrodes which engage the wall of the borehole, as the mudcake thickness increases, more and more of the electrode current is short 3,379,965 Patented Apr. 23, 1968 circuited back to the borehole current return point by the relative low resistance path formed by the mudcake. This makes the measurements of the flushed zone resistivity and thus, any subsequent formation porosity determination more difficult, because the measurement is influenced to a large degree by the mudcake resistivity. This problem is overcome to a large extent by using a wall-engaging electrode system of the focused type. Systems of this type are to be found in US. Patent No. 2,712,629, granted to H. G. Doll on July 5, 1955, and in US. Patent No. 3,132,298, granted to H. G. Doll and J. L. Dumanoir on May 5. 1964. In a wallengaging electrode system of the focused type, a current how is emitted by one of the electrodes and used for determining or surveying the formation resistance characteristics. This surveying current flow is constrained to a desired lateral flow pattern by a focusing current emitted by another one of the electrodes which is adjacent to the main surveying electrode. Focusing current emitted by means of the focusing current electrodes opposes any tendency of the survey current to flow in an undesired direction, as for example, along the mudcake. Thus, most of the survey current is caused to penetrate laterally into the earth formations for some appreciable distance. As indicated by the Doll and Dumanoir patent, more elaborate precautions are required as the environment becomes more severe or the requirements on the measurements become more demanding.

The electrode system described in the Doll and Dumanoir patent provides an improved measurement of the resistivity of the invaded zone region of the earth formations even for a case of relatively thick mudcake. Among other things, this is accomplished by utilizing relatively large electrode surfaces on the wall-engaging pad member. It has been found to be especially desirable to extend the focusing electrode outward in a circumferential direction around the borehole on both sides of the central survey electrode. It has been found, however, that in some boreholes, undesired variations appear in the measurements. After considerable effort, it has been found that the undesired variations in these instances are caused primarily by relatively large changes or variations of the shape or contour of the borehole wall. If the crosssectional shape of the borehole remains fairly circular and the diameter fairly constant, results are satisfactory. If, however, a section of the borehole assumes an elliptical or other non-circular shape and if, at the same time, a fairly thick mud-cake is present on the borehole wall, then the measured value will be somewhat different depending upon what side of the ellipse is being engaged by the electrode system.

It has been found that this undesired effect can be largely eliminated if a constant relationship can be maintained between the curvature of the borehole wall and the curvature of the wall-engaging face of the electrodes. This requires providing different electrode system curvatures for the different sides of the ellipse, depending upon which side of the ellipse the electrode system is pressed against.

The relationship between the formation resistivity and the measurement made with an electrode system is described by the formula:

where R is the formation resistivity, K is a proportionality constant or calibration constant depending upon the geometry of the electrode array including the size and shape of the individual electrodes and the distance between the electrodes, V the voltage at one of the electrodes, and I the current emitted by one of the electrodes. Either the voltage may be kept constant and current variations measured, or the current kept constant and the voltage variations measured. The curvature of the electrode device is a factor in determining the K coefficient in the above formula. Therefore, changes in the electrode system curvature will tend to introduce corresponding changes into the measured values. Since these changes are independent of the formation resistivity, they are undesirable.

It is an object of the present invention therefore, to provide new and improved apparatus for measuring electrical resistance properties of subsurface earth formations adiacent to a borehole.

It is another object of the present invention to provide new and improved wall-contact electrode apparatus of the focused type which provides improved focusing action under adverse borehole conditions.

It is a further object of the present invention to provide a new and improved focused type wall-contact electrode system which can adapt itself to excessive variations in radius of curvature of the borehole wall.

It is an additional object of the invention to provide a new and improved focused type wall-contact electrode system which can adapt itself to excessive changes in the curvature of the borehole wall without introducing excessive changes in the calibration constant of the system.

In accordance with the present invention, apparatus for investigating earth formations comprises a flexible pad member having an exploring means for investigating the earth formations and adapted to be pressed against a borehole wall, the pad member having a relatively stiff central portion carrying part of the exploring means. The pad member also has a relatively stiff portion adjacent to the central portion and carrying another part of the exploring means. The pad member additionally has a hinging means, including fluid-filled displaceable means longitudinally disposed between the central and adjacent pad portion for rotating the adjacent pad portion with respect to the central pad portion for enabling the flexible pad member to more nearly conform to the curvature of the borehole wall.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.

Referring to the drawings:

FIG. 1 illustrates, partially in cross-section, a representative embodiment of borehole investigating apparatus constructed in accordance with the present invention;

FIG. 2 is an enlarged front view of the wall-engaging face of one of the electrode pads of the FIG. 1 apparatus;

FIG. 3 is a cross-sectional view of the electrode pad of FIG. 2 taken along the section line 33 thereof;

FIG. 4 is a partial cross-sectional view of the electrode pad of FIG. 2 taken along the section line 44 of FIG. 5 showing a hinging means therein; and

FIG. 5 is a cross-sectional view of the electrode pad of FIG. 2 taken along the section line 5-5 thereof; and

FIG. 6 is a typical cross-sectional view of the electrode pad of FIG. 2, the electrode pad being disposed in a borehole with current flow lines indicated therein, and an electrical schematic of the circuitry of the present invention.

Referring to FIG. 1 of the drawings, there is shown a representative embodiment of a downhole portion of an apparatus constructed in accordance with the present invention for investigating a formation 10 traversed by a borehole 11, the borehole 11 being filled with a conductive fluid or drilling mud 12. This apparatus includes an elongated support member 13 adapted for movement through the borehole 11. The support member 13 includes an upper instrument housing portion 14 of a generally cylindrical shape having a hollow fluid-tight interior for enclosing certain downhole electrical circuits to be discussed hereinafter. The support member 13 also includes an intermediate frame or chassis portion 15 in the form of an iron or steel I-beam or equivalent structure and a lower nose portion 16 of a generally cylindrical shape.

Supported from an intermediate I-beam portion 15 of the support member 13 on opposite sides thereof are a pair of electrode pad members 17 and 18, each of which is adapted to be urged against the wall of the borehole 11. The pad member 17 is supported by way of support arms 19 and 29, which are pivotally coupled to the pad member 17 and the I-beam portion 15. Similarly, pad member 18 is supported by Way of support arms 21 and 22, which are pivoted to both the pad member 18 and I-beam portion 15. A suitable actuating mechanism for extending and retracting support arms 19 and 21 is included within the lower portion of the instrument housing section 14.

The downhole portion of the apparatus also includes a current return electrode B located on the support member 13 close enough to the location of the pad member 17 so as to be electrically proximate thereto, but longitudinally spaced apart from the location of the pad member 17 so that no portion of this B electrode is located directly behind the pad member 17. This B current reture electrode is of generally cylindrical shape and is mounted on suitable electrical insulation material which either covers or forms the nose portion 16 of support member 13. Electrical connection to the B current return electrode is made by way of an insulated conductor 26 which extends upwardly along the I-beam portion 15 to the instrument housing 14.

The downhole support member 13 is suspended in the borehole 11 by Way of an armored multi-conductor cable 27 which extends upwardly to a suitable drum and winch mechanism (not shown), located at the surface of the earth for raising and lowering the support member 13. The first hundred feet or so of the cable 27 immediately above the instrument housing portion 14 is covered with a layer of electrical insulation material 33 such as rubber. Supported on this layer of insulation material 33 towards the upper end thereof are an electrically remote current return electrde B and an electrically remote potential reference electrode N.

The electrical connections to electrodes A M, and A are made by way of insulated conductors 25, 28, 29 and 30. These insulated conductors 25, 28, 29 and 30 pass upwardly through the hollow interior of the support arm 19 to the electrical circuits contained within the instrument housing portion 14, or if support arm 19 is not hollow, as for example, a channel shaped member, then conductors 25, 28, 29 and 30 are secured to support arm 19 to pass upwardly to instrument housing portion 14. Mechanical connections of the electrode pad member 17 to the support arms 19 and 20 is made by way of lug members 31 and 32 respectively. The electrical continuity of the lug members 31 and 32 may be broken by means of non-conductive inserts.

Only the electrode pad member 17 will be dealt with in the explanation of this invention. The pad member 18 can be any downhole testing apparatus, the particular apparatus not being a feature of this invention.

Considering the electrode pad member 17, the FIG. 2 view of electrode pad member 17 shows the face portion of the pad that is pressed against the borehole wall. The major portion of the face of pad member 17 is formed by a focusing electrode A which forms most of the outer portion of pad member 17. Focusing electrode A has a non-flexible main central portion 34 and non-flexible adjacent wing portions 35 immediately adjacent to and on opposite sides of main central portion 34. Both portions of focusing electrode A are made of a suitable electrically conductive material, such as iron. The main central portion 34 and adjacent wing portions 35 of focusing electrode A are connected together by a series of flexible metal braids 36 (shown as dotted lines) to provide electrical continuity between the portions of focusing electrode A The flexible metal braids 36 are riveted to portions 34 and 35 of focusing electrode A The electrode pad member 17 also includes an elongated non-flexible electrode A of rectangular shape centrally located inside of central portion 34 of focusing electrode A Electrode =A constitutes a survey current electrode for emitting survey current into the adjacent earth formations. Located intermediate of survey electrode A and focusing electrode A is a non-fiexible potential monitor electrode M. This potential monitor electrode M has an exposed surface portion of narrow width which defines a path encircling the A survey electrode. Electrodes A M and A are embedded in a suitable insulation material 37 such as rubber. Insulation material 37 has a portion 37a encircling survey electrode A which insulates survey electrode A from potential monitor electrode M, and a portion 37b between potential monitor electrode M and central portion 34 of focusing electrode A which insulates potential monitor electrode M from focusing electrode A1. The electrodes A M and A are bonded in place during the molding process.

Referring now to FIG. 3, there is shown a cross-sectional view of electrode pad member 17 taken along the section line 33 of 'FIG. 2. Survey electrode A of pad member 17 is centrally located with respect to the other electrodes of pad member 17 and is embedded in rubber insulation material 67. The survey electrode A is made of a non-flexible conductive material, such as iron. Adjacent to survey electrode A on both sides thereof is the monitor electrode M, also made of a non-flexible conductive material, such as iron. On the upper and lower part of the electrode portion of electrode pad member 17 and adjacent to monitor electrode M is central portion 34 of focusing electrode A also made of a non-flexible conductive material, such as iron.

Also embedded in insulation material *37 on the opposite side of pad 17 from the electrodes is a support plate 38. Forming a part of support plate 38 are lug members 31 and 32, to which support arms 19 and 20 are attached (see FIG. 1). Conductors 25, 28, 29 and 30 are embedded in insulation material 37 and connected to electrodes A M and central portion 3 4 of A by screws 39. All of the metal members of electrode pad member 17 are bonded in insulation material 37 during the molding process.

Looking now at FIG. 4, there is shown a partial crosssectional view showing a cutaway view of the hinging means of pad 17. Insulation material 37 has a cylindrical hollow chamber 40 formed therein and running longitudinally through pad member 17 and located between nonflexible portions 34 and 35 of focusing electrode A and at a fixed distance from the wall-engaging face of electrode pad member 17. Located on both ends of the cylindrical hollow chamber are circular passageways 41 formed in insulation material 37. Circular passageways 41 are open to the outside of pad member 17 and thus provide access to cylindrical hollow chamber 40 within insulation material 37.

Located within cylindrical hollow chamber 40 is a bladder or sack 42 made of an elastic material, such as rubber. Bladder 42 is filled with a suitable fluid 43 having a low temperature coefficient as, for example, silicone oil. As an alternative, cylindrical hollow chamber 40 could be filled with certain materials in place of bladder 42, such as very soft rubber or plastic. One end of bladder 42 has a metal insert 44 placed in an opening in bladder 42. Metal insert 44 may be removed to enable removing the fluid 43 located within bladder 42 through circular passageway 41. There is sufficient space left on both sides of bladder 42 within cylindrical hollow chamber 40 to allow bladder 42 to stretch outwardly in both directions. The force of fluid 43 within bladder 42 maintains the ends of bladder 42 approximately equal distance from the ends 56 of cylindrical hollow chamber 40. The bladder 42 10- cated within cylindrical hollow chamber 40 is also located on the other side of pad member 17 in the same location between portions 34 and 35 of focusing electrode A that is, pad member 17 is symmetrical as concerns the fluidfilled bladder 42 located within cylindrical hollow chamber 40 shown in FIG. 4.

Looking now at FIG. 5, there is shown a cross-sectional view of pad member 17 taken along cross-section line 55 of FIG. 2. Electrodes A M and A are shown located on the wall-engaging face of pad member 17. Flexible metal braids 36 are connected to the non-flexible electrode portions 34 and 35 of focusing electrode A by rivets 45, shown on both sides of survey and monitor electrodes A and M. Metal braid members 36 provide electrical continuity between electrode portions 34 and 35 of focusing electrode A and still allow for bending between non-flexible electrode portions 34 and 35. The ends of non-flexible electrode portions 35 are turned down at the ends and embedded in insulation material 37, except where flexible metal braids 36 are connected. Located intermediate of and below non-flexible electrode portions 34 and 35 of focusing electrode A on both sides of pad member 17 are bladders 42 located within cylindrical hollow chamber 40 and filled with fluid 43. The metal support member 38 of which lug members 32 form a part thereof is shown embedded in insulation material 37. It can be seen that insulation material 37 insulates the electrodes A M and A from one another, while at the same time, holding these electrodes along with the other items of pad member 17 in place within pad member 17.

Looking now at FIG. 6, there is shown a typical crosssectional view of pad member 17 pressed against a mudcake 46 lining the borehole wall 11. There is shown a representative electrical schematic associated with pad member 17 and the current flow lines emanating into the earth strata surrounding the borehole from pad member 17.

Turning now to the schematic diagram portion of FIG. 6, the circuit portion contained within the dash line box 14 corresponds to the circuit portions that are located within the interior of the instrument housing portion 14 of FIG. 1. The electrical connections to the electrodes on pad member '17 are shown in FIG. 3, but represented in FIG. 6 as conductors touching electrodes A, M and A for purposes of clarity of the electrical circuits.

The circuits are constructed to maintain the potential level V of the monitor electrode M very nearly constant with respect to a remote reference point and, at the same time, to determine the formation resistance characteristics by measuring the resulting variations in the magnitude of the survey current I emitted from the central A electrode. Alternating-current reference voltage V is supplied from source 47 by way of input transformer 48, a highgain amplifier 49, and a conductor 28 to the A focusing current electrode. Also supplied to the input transformer 48 of the amplifier 49 via conductor 29 is a signal representative of the potential, V of the potential monitor electrode M. This potential signal V is supplied with like polarity to the side of the transformer 48 primary winding opposite from the reference voltage V,, so that if the monitor electrode M is at a potential nearly equal to the reference V then the net input signal to the amplifier 49 is nearly zero. If, on the other hand, the potential level V of the monitor electrode M differs from the V value, then the amplifier 49 input signal, which is supplied to the A electrode so as to bring the monitor voltage V back to the desired V value. Thus, the potential level of the monitor electrode M is kept substantially constant with respect to the remote reference point represented by electrode N.

In addition, the potential difference between monitor electrode M and the A survey current electrode is adjusted so as to be maintained at a value of substantially zero. To this end, the monitor electrode signal V is also supplied to an input transformer 50 of a high-gain amplifier 57. A signal representative of the potential level of the A electrode is supplied to the opposite side of the primary winding of the input transformer 50. This A signal is supplied by way of conductor 52. Thus, if the M-A potential difference is not substantially zero, then the error signal existing at the input of amplifier 51 serves to adjust the amplifier output current i which is supplied to the A electrode by way of conductors and 3t? to establish this desired Zero potential difference.

As the formation resistivity in front of the pad member 17 varies, different amounts of survey current i will be required to maintain the M-A potential difference at a zero value. The magnitude of this survey current required will be directly proportional to the electrical conductivity of the formation material. This indication is transmitted to the surface of the earth by means of transformer 53 having a low impedance primary winding coupled in series between amplifier 51 and conductors Z5 and 3% leading to the A electrode. Thus, the voltage signal appearing across the secondary winding of transformer 53 is proportional to the I current flow and therefore to the formation conductivity. This voltage signal is transmitted to a galvanometer unit at the surface of the earth by way of cable conductors 55 and 56.

Instead of using the constant voltage circuit of FIG. 6, a constant current circuit such as shown in US. Patent No. 3,132,298, supra, could be used. As a further alternative, both the voltage and current may be varied and a suitable ratio circuit or device used for taking a ratio of voltage to current or vice versa for providing the desired output signal.

Electrode pad member 17 is shown pressed against a borehole wall 11 lined with a mudcake 46 in FIG. 6. The dotted line representation 17a of pad member 17 indicates the free space shape of pad member 17, that is, the shape of pad member 17 if there were no borehole wall. The solid line representation 17 indicates the shape of pad member 17 in the borehole when the radius of curvature of the borehole is less than the radius of curvature of pad member 17. It can be seen from FIG. 6 that the outer wing portions of pad member 17 which include non-flexible portion 35 of focusing electrode A rotates about the cylindrical hollow chamber 4-9 for enabling the pad member 17 to more nearly conform to the radius of curvature of the borehole wall 11. The entire outer wing portions of pad member 17 on each side of the central portion of pad member 17, which wing portions each include the metal electrode portions 35 as well as the insulation material 37 behind electrode portions 35, must each rotate as a single non-flexible unit because of the structural non-flexibility that electrode portions 35 give to each win portion. In addition, the central portion of pad member 17, which comprises the electrodes A M, and portion 34 of A insulation material 37, and the support plate 38 on the back of pad member 17, is a single nonv flexible pad portion because of the structural non-flexibility of portion 34 of focusing electrode A Thus, it can be seen that there are three non-flexible portions of pad member 17, i.e., one central portion and two adjacent Wing portions. Located between the non-flexible central pad portion and each non-flexible adjacent pad portion are two hinging means, which include cylindrical hollow chamber 40, fluid-filled elastic bladder 42, fiexible metal braids 36 and that portion of insulation material 37 immediately surrounding hollow chamber 40.

Since cylindrical hollow chamber 40 is hollow, the nonfiexible adjacent wing portions of pad member 17, including electrode portions 35 of focusing electrode A on both sides of pad member 17, are able to rotate about cylindrical hollow chamber dil to a much greater extent than if this pivot point were solid rubber. Thus, by providing a hollow cylindrical portion running longitudinally with respect to the borehole through pad member 17 on both sides of central non-flexible portion of focusing electrode A and between non-flexible portions 35 thereof, the natural flexing action of the insulation material 37 is greatly enhanced and the non-flexible portions of focusing electrode A are able to rotate to a greater extent.

However, when pad member 17 is in a borehole with a larger radius of curvature than the radius of curvature of pad member 17, it is desirable for the non-flexible portions 35 of focusing electrode A to be urged towards the borehole wall. However, cylindrical hollow chamber 4'17, while augmenting the flexibility of pad member 17, at the same time, decreases the ability of non-flexible electrode portions 35 of focusing electrode A to spring back to the position shown as 17a in FIG. 6. It would be desirable therefore to provide a hollow chamber to augment the flexin ability of electrode pad member 17, while at the same time, providing a great degree of springing ability as if the hollow chamber were not present, so that pad member 17 will more nearly conform to the radius of curvature of the borehole wall when that radius of curvature is greater than the central non-flexible portion of electrode pad member 17.

Looking now at FIGS. 4 and 6 in conjunction, the fluid-filled bladder 4-2 does not extend to the ends 56 of cylindrical hollow chamber When the radius of curvature of the borehole wall decreases, thus exerting pressure on the ends of non-flexible portions 35 of focusing electrode A thus causing adjacent pad portions 35 to rotate, the cylindrical hollow chamber decreases in diameter, thus decreasing the unit volume thereof. The cylindrical hollow chamber 49 will take the non-cylindrical shape shown by the solid line representation of hollow chamber in FIG. 6. This decrease in diameter of cylindrical recess h; causes the fluid 43 within bladder 42 to press outwardly against the end portions of bladder 42. This squeezing action on the fluid 4-3 within bladder 42, then, causes the bladder 42 to stretch outwardly towards the end portions 56 of hollow chamber When the radius of curvature of that portion of borehole wall 11 that pad member 17 is pressed against begins increasing, the fluid-filled elastic bladder $2 tends to decrease in length causing the fluid 43 within bladder 42 to exert a pressure against the sides of hollow chamber til to force it back to its original cylindrical shape. This force against the sides of hollow chamber 40 causes the non-flexible adjacent wing portion 35 of focusing electrode A to rotate towards the borehole wall. Thus, it can be seen that by placing a fluid-filled bladder 42 Within a cylindrical hollow chamber 49, the ability of non-flexible adjacent wing portions 35 of focusing electrode A to rotate in conformity with the radius of curvature of the borehole wall is greatly enhanced and, at the same time, the springing action is sufficient to force these non-flexible portions of focusing electrode A against the borehole wall for a wide range of radius of curvatures of the borehole wall.

However, while the focusing electrode A changes shape substantially in conformance with the shape of the borehole, it can be seen that the solid non-flexible portion of pad member 17, which includes electrodes A M and non-flexible portion 34 of A do not change shape and thus part of A and M may be disposed at a small distance from the mudcake 46. It has been found, however, that it is not nearly as critical to have all of the survey electrode A and the monitor electrode M in contact with the rnudcake 46 as it is to have at least a part of focusing electrode A on either side of survey electrode A in contact or close proximity with the mudcake 46. When survey electrode A is not in contact with the mudcake 46, the potential set up by focusing electrode A, will not allow the survey current emitted from survey electrode A to diverge. On the other hand, when focusing electrode A is at a distance from mudcake 46, the focusing current emitted therefrom will diverge rapidly back to the mudcake since there is no potential outside of focusing electrode A to keep it from doing so.

Now concerning the focusing action of the present invention, there are seen current flow lines I and I; in FIG. 6. Survey current electrode A emits survey current I, into the earth formation surrounding the borehole, which current diverges back to the current return electrode B (see FIG. 1). The divergence of the survey current I back to the current return electrode B becomes more severe when the mudcake 46 lining the borehole wall 11 is thick, in which case some of the survey current I will flow through the mudcake 46 to the current return electrode B However, the focusing current I; emitted from focusing current electrode A directs the survey current I, deeper into the earth formation by setting up a potential gradient concentrically surrounding survey current I which maintains survey current I in a radial flow away from the borehole.

However, when the mudcake 46 is very thick and the borehole radius of curvature becomes non-uniform around the circumference of the borehole, a pad member of non-flexible construction could not conform to the radius of curvature of the borehole wall. If a pad member of non-flexible construction adapted for small radius of curvature boreholes were placed on that portion of the borehole wall where the radius of curvature of the borehole wall is greater than the radius of curvature of the pad member and the mudcake thickness fairly substantial, some of the focusing current I; emitted from focusing electrode A would tend to diverge more quickly back through the mudcake 46 to the mud-filled portion of the borehole proper. Thus, the survey current I, emitted from survey electrode A would also tend to diverge too quickly. On the other hand, when the radius of curvature of the borehole wall is less than that of a nonfiexible pad member adapted for large radius of curvature boreholes, the survey electrode A will be disposed at some great distance from the borehole wall since the focusing electrode portion of the pad member will contact the borehole wall first. In this event, the survey current emitted from survey electrode A will have to travel through the mud 12 in the borehole 11 for an appreciable distance. Since the mud 12 may have a different resistivity than the resistivity of the surrounding earth formations the resistivity or conductivity measurements will be in error by an amount determined by the difference in resistivity of the mud with the surrounding earth formations and the distance that survey electrode A is disposed from the borehole wall 11.

However, with the flexible features of the present electrode pad member 17, the survey electrode A will always be pressed against the mudcake 46 lining the borehole wall 11 and the non-flexible outer portion 35 of focusing electrode A will be rotated so that at least the end portion thereof will be in contact to the mudcake 46. Thus, it can be seen that through the action of fluidfilled bladder 42 located within cylindrical hollow chamber 40, the pad member 17 will more nearly conform to the radius of curvature of the borehole wall for very large variations in the radius of curvature of the borehole wall.

As seen by the current flow lines of FIG. 6, focusing current I, emitted from focusing electrode A extends laterally outward into the earth because the focusing electrode A is in close proximity to the mudcake 46 lining the borehole wall 11. Thus, there is very little divergence of the focusing current I, emitted from points on the focusing electrode A which are nearest to the survey electrode A which in turn, extends the survey current I emitted from survey electrode A laterally outward from the borehole for an appreciable distance. The focusing current I, emitted from the end points of focusing electrode A, will tend to keep the focusing current I, emitted from the points on focusing electrode A closest to survey electrode A extending laterally outward into the earth, which in turn, keeps the survey current I extending laterally outward into the earth. Thus, it can be seen that there is very little divergence of the survey current I under adverse borehole conditions and the accuracy of the resistivity or conductivity measurements will be improved under such conditions.

When the physical configuration of an electrode pad member is varied, the proportionality constant K of the device will vary. However, with the novel form of construction of electrode pad 17 shown in the present invention, there will be very little variation in the proportionality constant K. One reason for the small variation in the proportionality constant K is that the electrodes on the front side of pad member 17 are electrically insulated from the back side of electrode pad 17 by the insulation material 37. Thus, because the front and back sides of electrode pad 17 are insulated from one another, a single value of proportionality constant K can be utilized in de termining the resistivity or conductivity of adjoining earth formations over a wide range of radius of curvatures of the borehole.

While there have been described what are at present considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member comprising:

(1) la non-flexible central portion having at least one electrode;

(2) non-flexible portions adjacent to the central portion and having at leastma portion of one electrode; and

(3) hinging means, including fluid-filled chamber means, located within the flexible pad member for rotating the adjacent pad portions with respect to the central pad portion for enabling the flexible pad member to more nearly conform to the curvature of the borehole wall.

2. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member comprising:

(1) a non-flexible central portion having at least one electrode;

(2) non-flexible portions adjacent to the central portion and having at least a portion of one electrode; and

(3) hinging means, including fluid-filled chamber means, located within the flexible pad member and between the central pad portion and the adjacent pad portions for rotating the adjacent pad portions with respect to the central pad portion for enabling the flexible pad member to more nearly conform to the curvature of the borehole wall.

3. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against .a borehole wall, the pad member comprising:

(1) a non-flexible central portion having at least one electrode;

(2) non-flexible portions adjacent to the central portion on opposite sides thereof and having at least a portion of one electrode;

(3) two hinging means, each including fluid-filled chamber means, located within the flexible pad member, one hinging means located on each side of the central pad portion between the 1 1 central pad portion and each adjacent pad portion, for rotating the adjacent pad portions with respect to the central pad portion for enabling the flexible pad member to more nearly conform to the curvature of the borehole wall.

4. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member comprising:

(1) a non-flexible central portion having at least one electrode;

(2) non-flexible portions adjacent to the central portion and having at least a portion of one electrode; and

(3) hinging means, including fluid-filled chamber means, located within the flexible pad member between the central pad portion and each adjacent pad portion and at a fixed distance from the borehole wall-engaging face of the flexible electrode pad for rotating the adjacent pad portions with respect to the central pad portion for enabling the flexible pad member to more nearly conform to the curvature of the borehole wall.

5. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member having:

(1) a non-flexible central portion having at least one electrode;

(2) a non-flexible portion adjacent to the central portion and adapted to rotate about the central pad portion and having at least a portion of one electrode;

(3) a hollow chamber located within the flexible pad member for augmenting the rotation of the adjacent pad portion; and

(4) fluid-filled means located within the hollow chamber for urging the adjacent portion toward the borehole wall.

6. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member having:

(1) a non-flexible central portion having at least one electrode;

(2) a non-flexible portion adjacent to the central portion and adapted to rotate about the central pad portion and having at least a portion of one electrode;

(3) a hollow chamber located within the flexible pad member between the central pad portion and adjacent pad portion and at a fixed distance from the wall-engaging face of the electrode pad for augmenting the rotation of the adjacent pad portion; and

(4) fluid-filled elastic bladder means located within the hollow chamber for urging the adjacent pad portion toward the borehole wall.

7. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member having:

(1) a non-flexible central portion having a portion of a first electrode and at least one other electrode;

(2) a non-flexible portion adjacent to the central portion and adapted to rotate about the central pad portion and having a portion of the first electrode;

(3) 1a flexible metal member connecting the portions of the first electrode;

(4) a hollow chamber located within the flexible pad member between the central pad portion and adjacent pad portion and at a fixed distance from the wall-engaging face of the electrode pad for augmenting the rotation of the adjacent pad portion; and

(5) fluid-filled elastic bladder means located within the hollow chamber for urging the adjacent pad portion toward the borehole wall.

8. Apparatus for investigating earth formations traversed by a borehole comprising:

(a) a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member having:

(1) a non-flexible central portion having at least one electrode;

(2) a non-flexible portion adjacent to the central portion and having at least one electrode;

(3) hinging means, including fluid-filled chamber means, located within the flexible pad member for rotating the adjacent pad portion with respect to the central pad portion for enabling the flexible pad member to more nearly conform to the curvature of the borehole wall;

(b) means for supplying a survey current to said at least one electrode within the central pad portion; and

(c) means for supplying a focusing current to said at least one electrode within the adjacent pad portion for maintaining the survey current in a radial direction from the borehole for a substantial distance.

9. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member having:

(1) a non-flexible central portion having at least one electrode;

(2) a non-flexible portion adjacent to the central portion and adapted to rotate about the central pad portion and having at least one electrode;

(3) a hollow chamber located within the flexible pad member between the central pad portion and adjacent pad portion and at a fixed distance from the wall-engaging face of the flexible pad member for augmenting the rotation of the adjacent pad portion; and

(4) fluid-filled elastic bladd er means, located within the hollow chamber and filling only a portion thereof so that the elastic bladder means may expand upon rotation of the adjacent pad portion, for urging the adjacent pad portion toward the borehole wall.

10. Apparatus for investigating earth formations traversed by a borehole comprising:

(a) a flexible pad member having an electrode array and adapted to be pressed against a borehole wall, the pad member having:

(1) a non-flexible central portion having a portion of a first electrode and at least one other electrode;

(2) a non-flexible portion adjacent to the central portion and adapted to rotate about the central pad portion and having a portion of the first electrode;

(3) a hollow chamber located within the flexible pad member between the central pad portion and adjacent pad portion and at a fixed distance from the wall-engaging face of the flexible pad member for augmenting the rotation of the adjacent pad portion;

(4) fluid-filled elastic bladder means, located within the hollow chamber and filling only a portion thereof so that the elastic bladder means may expand upon rotation of the adjacent pad 13 portion, for urging the adjacent pad portion toward the borehole wall;

() a flexible metal member connected to the portions of the first electrode;

(b) means for supplying a survey current to said at least one other electrode within the central pad portion; and

(c) means for supplying a focusing current to said first electrode for maintaining the survey cunrent in a radial direction from the borehole for a substantial distance.

11. Apparatus for investigating earth formations traversed by a borehole comprising:

a flexible pad member having exploring means for investigating earth formations and adapted to be pressed against the borehole wall, the pad member having:

(1) a relatively stiff central portion carrying part of the exploring means;

(2) a relatively stilt portion adjacent to the central portion and carrying another part of the exploring means; and

(3) hinging means including fluid-filled displaceable means longitudinally disposed between the central and adjacent pad portion, said displaceable means normally maintaining the curvature of said pad member at a given curvature but displaceable from its normal position upon force being applied to said adjacent portion to permit relative movement of the adjacent pad portion with respect to the central pad portion so that the pad member may more nearly conform to the curvature of a borehole wall.

12, The apparatus of claim 11 wherein the displaceable means is a fluid-filled bladder housed in a hollow chamber, the fluid-filled bladder being longitudinally displaceable in response to force applied on the adjacent pad portion to allow the adjacent pad portion to pivot with respect to the central pad portion.

13. Apparatus for inventigating earth formations traversed by a borehole comprising:

a flexible pad member having exploring means for investigating earth formations and adapted to be pressed against the borehole wall, the pad member having:

(1) a relatively stiif central portion carrying part of the exploring means;

(2) a relatively stilt portion adjacent to the central portion on one side thereof and carrying another part of the exploring means;

(3) a relatively stiff portion adjacent to the central portion on the other side thereof and carrying another part of the exploring means; and

(4) hinging means including fluid-filled displaceable means longitudinally disposed between the central and adjacent pad portions, said displaceable means normally maintaining the curvature of said pad member at a given curvature but displaceable from its normal position upon force being applied to said adjacent portion to permit relative movement of the adjacent pad portion with respect to the central pad portion so that the pad member may more nearly conform to the curvature of a borehole wall.

14. Apparatus for investigating earth formations traversed by a borehole comprising:

(a) a central support member adapted for movement through the borehole;

(b) a flexible pad member having exploring means for investigating earth formations and adapted to be pressed against the borehole wall, the pad member having:

(1) a relatively stifl? central portion carrying part of the exploring means;

(2) a relatively stiff portion adjacent to the central portion and carrying another part of the exploring means;

(3) hinging means including fluid-filled displaceable means longitudinally disposed between the central and adjacent pad portion, said displaceable means normally maintaining the curvature of said pad member at a given curvature but dis-l placeable from its normal position upon force being applied to said adjacent portion to permit relative movement of the adjacent pad portion with respect to the central pad portion so that the pad member may more nearly conform to the curvature of a borehole wall; and

(c) means movably connecting the relatively stiff pad portion to the central support member and adapted to urge the pad member against the borehole wall.

References Cited UNITED STATES PATENTS 2,688,115 8/1954 Hildebrandt 32410 2,732,525 1/1956 Blanchard et al. 324l0 2,961,600 11/1960 Tanguy 324l0 XR 3,132,298 5/1964 Doll et al. 324-10 RUDOLPH V. ROLINEC, Primary Examiner.

G. R. STRECKER, Assistant Examiner. 

